Flue gas desulfurization (FGD) processes currently in use today typically employ wet calcium-based technology to remove sulfur from various flue gas sources. Sulfur is absorbed from the flue gas as SO.sub.2 into a calcium-containing liquid phase and forms calcium sulfite or calcium bisulfite. At least some of calcium sulfite or bisulfite will be oxidized to calcium sulfate. The amount of calcium sulfate produced will depend, in large measure, on the amount of excess oxygen in the flue gas. Crystals of calcium sulfite or calcium sulfate, primarily as gypsum (CaSO.sub.4.2H.sub.2 O), will form as the respective critical relative saturations for each of the compounds is exceeded in the liquid phase. If the ratio of the amount of sulfite oxidized to sulfate compared to the total amount of sulfur compounds absorbed from the flue gas is less than 15 to 18%, all of the sulfur will be purged from the FGD system as a coprecipitate within the calcium sulfite crystal matrix. If this occurs, the calcium sulfate or gypsum relative saturation in the liquid phase will never exceed 1.0, and, therefore, calcium sulfate or gypsum precipitation cannot occur.
The widespread use of calcium-based wet scrubbers for controlling SO.sub.2 emissions from utility boilers and the like generates as waste quantities of calcium sulfate and/or calcium sulfite solids in the range of 5 to 10 million tons annually in the United States. This material is currently disposed of primarily in ponds and landfills. The volume of this FGD waste material and the disposal methods currently employed depends largely on the chemical composition of the precipitated solids. Calcium sulfate dihydrate (gypsum) solids are generally larger and more regularly shaped than are calcium sulfite hemihydrate solids. While gypsum can usually be dewatered to produce about 85% solids, only about 50 to 70% calcium sulfite hemihydrate solids can typically be produced. Moreover, the calcium sulfite sludges tend to exhibit thixotropic-like behavior, which increases the difficulty and expense of their disposal.
The size and shape of calcium sulfite crystals produced in the wet calcium-based FGD processes can affect the dewatering and handling characteristics of the resulting waste product. The performance of the dewatering equipment can be affected significantly by the size and shape of these crystals. If the crystals or particles are smaller than about 5 microns, they will impede filtration by blinding the filter media or forming a fine film on the solids being filtered, thus reducing the filtration rate and increasing the amount of moisture retained by the calcium sulfite or gypsum. Additionally, if the solids produced by FGD processes is to be a useful product, the ability to control the particle size, moisture content and impurity level is critical.
The calcium sulfite solids disposal problems could be reduced by modifying available FGD processes to produce larger crystals. FGD process sludges could be dewatered more easily and with greater efficiency if the size and shape of the crystals and particles was larger and more regular. In addition, new systems could be designed with smaller, less expensive dewatering and disposal equipment.
The prior art has proposed solutions to the gypsum scale and calcium sulfite/calcium sulfate disposal problem in flue gas desulfurization systems. U.S. Pat. No. 4,342,733 to Steelhammer et al. discloses a method of inhibiting sulfite oxidation in scrubber liquor of the type containing sulfite species. A "topping" agent, which may be a polyphosphate or an organophosphonate or a mixture of these two types of compounds is added to the scrubber liquor to supplement and enhance the performance of specified sulfite antioxidants such as linear polyethyleneamines and substituted secondary and tertiary aromatic amines. This patent nowhere suggests the use of either polyphosphates or organophosphates to control crystal growth generally or calcium sulfite crystal size and shape or crystal habits specifically.
U.S. Pat. No. 4,818,506 to Lin et al. discloses the use of organophosphonates as gypsum scale inhibitors. Lin et al. does not suggest the use of organophosphonates to control calcium sulfite crystal size or shape in an inhibited oxidation environment in a wet calcium-based FGD system which is operating subsaturated with respect to gypsum.
The prior art, therefore, has failed to provide a method of treating wet calcium-based flue gas desulfurization process byproduct solids under inhibited oxidation conditions which controls the size and shape of the calcium sulfite crystals in these solids. A need exists, therefore, for a method of treating wet calcium-based FGD byproduct solids under inhibited oxidation conditions which permits control of the size and shape of the calcium sulfite and other crystalline solids produced by this process.